Many tungsten carbide hardfacing compositions useful for deposition by the plasma-transferred-arc process are known in the art. The aforementioned compositions have been applied as facings to prevent premature wear on mining, agricultural and plastic extrusion molding equipment.
The types of tungsten carbide bearing hardfacing compositions of the prior art are typified by having matrices of approximately the composition chromium 13.5% by weight, carbon 0.75% by weight, silicon 4.25% by weight, iron 4.75% by weight, boron 3.0% by weight with nickel comprising the balance. The amounts of tungsten carbide present in these blends typically range from 30 to 50% by weight. The overlays produced by plasma-transferred-arc deposition of these blends generally exhibit matrix hardnesses measured by the Rockwell C hardness test in the 50 to 60 range with approximately 80 to 85% retention of carbides from the starting composition.
Also, the presence of large amounts of chromium in the aforementioned matrices can disadvantageously lead to the formation of coarse, acicular chromium carbides (depending upon the welding conditions) with their corresponding detrimental effect upon impact resistance. More typically, the chromium combines with the boron present to form chromium borides which also disadvantageously contribute to brittleness in the matrix alloy. Since the matrix alloys above-described are inherently poor in impact resistance due to the relatively high chromium content, carbide additions have been kept at or below 50% by weight so as not to exacerbate this condition. However, the chromium has been considered necessary in prior art powders because of its hardening quality and its ability to enhance fluidity. That is to say, the chromium acts as a hardening factor in the matrix alloy and helps sustain the flow ability of the molten material for deposition. As a result, the poor impact resistance and brittleness of the matrix alloys is only worsened by the addition of more carbide; particularly with regard to sheer loading. Moreover, the increased brittleness results in the formation of cracks and pores in the overlays, which become even more prevalent with increased overlay thicknesses or number of layers.
Other prior art hardfacing powders are also known in the art for spray deposition by thermal spraying processes. These thermal spray powders are fundamentally different from welding powders, such as applied by the aforementioned plasma-transferred-arc welding process, because the thermal spray processes simply spray molten alloy onto a substrate for coating without metallurgical bonding. These processes are typified by the use of a flame to melt the powders which are limited to the lower available temperatures associated with a particular flame. Thus, without metallurgical bonding, the hardface overlays can be easily broken away from the substrate by an impact. Furthermore, the powders used for thermal spraying generally include melting alloys to assist in melting at the lower temperatures and fluxing agents in an attempt to increase bonding strength between the overlay and the substrate. The result is the production of a softer and less impact-resistant overlay. In contradistinction, a welding process forms a metallurgical bonding between the molten alloy of the overlay by forming a weld pool on the surface of a substrate of sufficient temperature to metallurgically bond with the substrate metal. This can be accomplished with the heat generated in a plasma-transferred-arc welding process. As a result, welded overlays cannot be easily broken free of the substrate on impact.
One known attempt at increasing bonding strength in a thermal spray powder includes an amount of nickel-aluminide in the composition. The nickel-aluminite actually creates more heat than the thermal sprayer, and this increased heat is used in bonding. However, the heat generated still falls short of the welding processes.